1. Field of the Invention
The present invention relates to a solid-state radiation detector having a charge storing section which stores the charges of the quantity corresponding to the dose of the projected radiation as latent image charges, and being capable of recording radiation image information as a static latent image in the charge storing section.
2. Description of the Prior Art
To reduce the dose of exposure of the subject and improve the performance of diagnosis in medical radiation photographing, etc., a method has been known up to now which uses, as a photosensitive material, a solid-state radiation detector (a static recorder) with a photoconductor, such as a selenium plate, responding to radiation, such as X-rays; projects the X-rays onto the detector, and stores the charges of the quantity corresponding to the dose of the projected radiation in the charge storing section of the detector as latent image charges for recording of radiation image information as a static latent image (a charge pattern); and uses a laser beam or line light to scan the detector to read out the radiation image information from the detector.
To efficiently store latent image charges in the charge storing section, a detector which is provided with microplates (minute conductive members) or an anisotropic conductive layer has been proposed (for example, U.S. Pat. Nos. 5,166,524, 4,535,468, 3,069,551, European Patent No. 0748115A1, which corresponds to Japanese Unexamined Patent Publication No. 9(1997)-5906, and Japanese Unexamined Patent Publication No. 6(1994)-217322).
The detector as proposed in the above-mentioned U.S. Pat. No. 5,166,524 is a detector with which conductive microplates having a size approximately equal to the smallest pixel size which can be resolved are provided on the surface of the detector, and these microplates form pixels in the fixed locations on the detector. When this detector is used to record a static latent image, and read out it, the electrode of a single plate which is to contact all the microplates is disposed on the surface of the detector; a voltage is applied to this electrode for subjecting it to an electric field; X-rays are projected onto it to store the static latent image in the charge storing section for carrying out the recording; and then the single-plate electrode is removed, and a signal is taken out from the microplates.
The detector is proposed in the above-mentioned U.S. Pat. No. 4,535,468 is a detector comprising a three-layer structure which has an X-ray photoconductive layer, a trap layer, and a photoconductive layer for reading in this order, the charge storing section for storing of charges generated in the X-ray photoconductive layer being formed by the trap layer. When this detector is used to record a static latent image and read it out, a high voltage is applied across the electrodes provided on both sides of the three layer structure, and X-rays are projected to store latent image charges in the charge storing section, and then the electrodes are short-circuited to read out the latent image charges.
The detector as proposed in the above-mentioned U.S. Pat. No. 3,069,551 is a detector with which an anisotropic conductive layer is provided in the detector, and the anisotropic conductive layer forms the charge storing section, and it is almost the same as that proposed in the above-mentioned U.S. Pat. No. 4,535,468.
The detector as proposed in the above-mentioned European Patent No. 0748115A1 is a detector with which a conductive microspot layer in which a number of microspots having a size much smaller than the pixel size are disposed is provided in the detector, and latent image charges are stored in the microspots.
Further, the detector as proposed in the above-mentioned Japanese Unexamined Patent Publication No. 6(1994)-217322 is a detector with which a conductive layer, an X-ray photoconductive layer, a dielectric layer, and an electrode layer comprising a number of microplates corresponding to the pixels are stacked, and a TFT (a thin film transistor) for reading out the charges is connected to each microplate. When the static latent image is to be read out from this detector, the TFTs are scanned and driven to read out the latent image charges stored in the charge storing section to the outside of the detector.
However, with the detectors as proposed in the above-mentioned U.S. Pat. No. 5,166,524 and Japanese Unexamined Patent Publication No. 6 (1994)-217322, pixels can be formed in the fixed locations on the detector by providing microplates as stated above, but with the detector of the U.S. Pat. No. 5,166,524, a single-plate electrode must be disposed on the surface of the detector for carrying out the recording, and thereafter, this single-plate electrode must be removed for taking out a signal, which means that the recording and reading operations are cumbersome, and with the detector of Japanese Unexamined Patent Publication No. 6(1994)-217322, it is necessary to provide TFTs for charge reading in the electrode layer comprising microplates, which presents a problem that the construction of the detector is complicated, resulting in the manufacturing cost for the detector being increased.
On the other hand, the detectors of the U.S. Pat. Nos. 4,535,468, 3,069,551 and European Patent No. 0748115A1 are detectors with which the charge storing section is formed by the trap layer or the like provided in the detector, but, the trap layer or the like is not such that charges are discretely stored for each pixel, presenting problems that pixels cannot be formed in the fixed locations, and the artifact (the structure noise) having a location dependency cannot be properly compensated for.
Also with this U.S. Pat. No. 4,535,468, etc., the electrode is a stripe electrode, and line light is used as reading light, the elements of the stripe electrode being scanned along the longitudinal direction of them with the line light. This means that for the direction of arrangement of the elements, pixels can be formed in the fixed locations, but because the elements are not divided by the pixel size along the longitudinal direction of them, pixels cannot be formed in the fixed locations for the longitudinal direction, resulting in an anisotropy in sharpeness being produced.
In addition, with any of the detectors as disclosed in the above documents, it is difficult to cause the latent image charges to have the same potential for each pixel, the latent image charge around the pixel cannot sufficiently be discharged, and the information around the pixel may not easily be taken out.
The solid-state radiation detector according to the present invention is a solid-state radiation detector having a first electrode layer, a photoconductive layer for recording which exhibits a conductivity when irradiated with radiation which has been projected or light emitted by excitation on the radiation, and a second electrode layer in this order, a charge storing section for storing the charges of the quantity corresponding to the dose of the radiation or the quantity of the light as latent image charges being formed in the vicinity of the surface of the photoconductive layer for recording, and radiation image information being recorded in the charge storing section as a static latent image,
in which a conductive member for causing the latent image charges to have the same potential is discretely provided in the charge storing section for each pixel for the static latent image, and is put in the electrically non-connected state.
Here, the phrase xe2x80x9cis provided for each pixelxe2x80x9d means that preferably one conductive member is provided for each pixel so that the charges around the pixel can be concentrated on the pixel central portion in reading by causing the latent image charges to have the same potential, and does not involve the style in which a number of conductive members are disposed at random for one pixel, and in reading the charges around the pixel cannot be concentrated on the pixel central portion.
The phrase xe2x80x9cis discretely provided, and is put in the electrically non-connected statexe2x80x9d means that the respective conductive members are disposed in the discrete state from one another, i.e., in the floating state in which they are not connected to one another, and that they are kept open even in the recording process and the reading process. When a plurality of conductive members are provided for one pixel, it is preferable that the members for one pixel be electrically connected to one another.
The size of this conductive member is preferably set at a value approximately equal to the pixel pitch. Alternatively, it may be set at a value smaller than the pixel pitch, say, less than half, and the conductive member may be disposed in the pixel central portion to concentrate the latent image charges on the pixel central portion. The size of this conductive member refers to the diameter for a circular conductive member, and the length of each side for a square conductive member. The shape of the conductive member may be any shape, such as a circle or a square.
The phrase xe2x80x9cthe vicinity of the surface of the photoconductive layer for recordingxe2x80x9d refers to the approximate boundary region between the photoconductive layer for recording and the other layer, including the region close to the boundary in the outer layer.
When the electrode of the first and/or second electrode layer of the radiation solid-state detector according to the present invention is a stripe electrode, the conductive member is preferably disposed so that it corresponds to the pixel locations which are defined by the stripe electrode.
The term xe2x80x9cstripe electrodexe2x80x9d means an electrode comprising a number of linear electrodes which are arranged. The term xe2x80x9clinear electrodexe2x80x9d means an electrode having a long and slender shape as a whole, and so long as it has a long and slender shape, it may of any shape, such as columnar or prismatic, but it is preferable that the linear electrode be particularly a flat plate electrode. To prevent the latent image forming process and the charge-recoupling process from being affected, such a measure as providing the linear electrode with holes having a desired shape, such as a circle or a square, in correspondence with the pixels, or with an elongated rectangular hole extending along the direction may be taken.
The sentence xe2x80x9cthe conductive member is disposed so that it corresponds to the pixel locations which are defined by the stripe electrodexe2x80x9d means that the conductive member is disposed so that it corresponds to the pixel locations which are defined by the way in which the linear electrodes of the stripe electrode are arranged. For example, when the electrode of either of the electrode layers is a strip electrode, the conductive member is disposed roughly just above or just under the linear electrodes. Further, when the electrodes of both electrode layers are stripe electrodes, and the linear electrodes of both electrode layers are disposed so that they are opposed to each other, the conductive member is disposed at locations where it is sandwiched between the corresponding two linear electrodes. Further, when the electrodes of both electrode layers are stripe electrodes, and the two linear electrodes are disposed so that they intersect each other, the conductive member is disposed at locations where the linear electrodes intersect. Further, when a new stripe electrode is to be provided in a layer other than the two electrode layers, for example, the photoconductive layer for recording, the conductive member is disposed at the sandwiched locations or the intersecting locations with respect to the new stripe electrode.
The solid-state radiation detector according to the present invention may have a charge transporting layer which acts roughly as an insulator for the latent image charges, and roughly as a conductor for charges opposite in polarity to the latent image charges, provided between the photoconductive layer for recording and the photoconductive layer for reading, with the charge transporting layer forming the charge storing section. In this case, it is preferable that the conductive member be provided at the boundary between the photoconductive layer for recording and the charge storing section.
The solid-state radiation detector according to the present invention may have a trap layer for catching the latent image charges, the trap layer forming the charge storing section. In this case, it is preferable that the conductive member be provided at the boundary between the photoconductive layer for recording and the trap layer.
The detector to which the conductive member according to the present invention is applicable may be of any type, provided that it is a detector which has a photoconductive layer for recording sandwiched between electrode layers, and may have a layer other than the photoconductive layer for recording. As the method for taking out the latent image charges to the outside of the detector for reading out the static latent image, the electric reading mode with which the charges are taken out according to the switch selection, and the light reading mode with which the charges are taken out by projecting an electromagnetic wave for reading onto the detector are available, and the detector to which the present invention is applicable may adopt either of these modes.
In carrying out the recording and reading of a radiation image with the use of the detector according to the present invention, the conventional recording method and device using a detector which is not provided with the conductive member according to the present invention can be utilized as they are, with no change required. When the present invention is applied to a detector of the light reading mode, it is preferable that the electromagnetic wave for reading be projected at least to the locations where the conductive member is provided.
With the radiation solid-state detector according to the present invention, the conductive member is discretely provided for each pixel in the charge storing section in the detector, which means that the latent image charges for each pixel stored on the conductive member can be caused to have the same potential, allowing the reading efficiency to be increased to a level higher than that obtained when the conductive member is not provided. This is because the latent image charges are held at the same potential in the range of the conductive member, so that the latent image charges around the pixel which are generally difficult to read out can be moved to the central portion of the conductive member, i.e., the pixel central portion, as the reading progresses, so long as they are in the range of the conductive member, which means that the latent image charges can be sufficiently discharged.
In addition, the pixel can be formed in the fixed location where the conductive member is disposed, which allows the structure noise to be compensated for.
Further, if the size of the conductive member is set at a value smaller than the pixel pitch, and disposed in the central portion of the pixel, the shape of the electric field formed in reading can be such that the electric field is attracted toward the conductive member, and so the latent image charges can be stored, being concentrated on the pixel central portion, which means the sharpeness of the image can be improved.
If this conductive member is provided, the latent image charges can be stored with no charge transporting layer or trap layer, and so device formation can be performed more easily.
When the detector having a charge transporting layer or trap layer is provided with the conductive member, the charge storing effect by such layer can also be utilized. In other words, if the size of the conductive member is set at a value smaller than the pixel pitch, and such a layer is not provided, a problem arises where the charges which have not been caught by the conductive member cannot be stored as latent images, resulting in the quantity of the stored charges being decreased, although they contribute to the improvement in sharpness. Contrarily, when such a layer is provided, the charges are caused to be stored as latent image charges, so that the sharpness can be improved without the quantity of stored charges being reduced.
Because the recording and reading can be carried out with the conductive member being kept in the floating state, there is no need to provide TFTs for reading out the charges on the conductive member, which makes it possible to obtain a simple construction detector with which the layers are stacked together, resulting in the manufacturing cost being lowered.
In addition, a device which is the same as the conventional device, which is not provided with the conductive member, can be utilized, meaning that, applying the present invention will not complicate the recording and reading methods and the device.
The present invention is intended to offer solid-state radiation detectors having pixels that can be formed at fixed locations on the detector, and the latent image charges around the pixel can be sufficiently discharged.
The present invention is also intended to offer detectors with which the sharpeness of the image detected can be improved.
Further, the present invention is intended to offer detectors with which no TFTs are used, resulting in the construction of the detector being simplified, and therefore an increase in manufacturing cost being avoided, and the recording and reading can be carried out in convenient ways.